1. Field of the Invention
This invention relates to an original reading device in an apparatus such as a facsimile transmitter or a digital copying apparatus in which original information is converted into electrical signals and a desired image is formed by the electrical signals.
2. Description of the Prior Art
In most original reading devices, an original image is converted into electrical signals by a solid state image pick-up element such as a charge coupled device (CCD). In such reading devices, when it is desired to increase the picture element density to be read on the surface of the original, for example, when it is desired to read 297 mm in the lengthwise direction of a JIS A4 original in 12 dots/mm or 16 dots/mm, the number of necessary picture elements of the solid state image pick-up element such as CCD is as great as 3564 in the former case and as great as 4752 in the latter case.
Now, the number of available picture elements of CCD is maximum 2048 and therefore, as known in Japanese Laid-open patent applications Nos. 102819/1979 and 102820/1979, a system in which a plurality of CCDs are arranged with respect to their electrical scanning direction, namely, the major scanning direction, is adopted.
In principle, the reading device of this type according to the prior art is as shown in FIG. 1 of the accompanying drawings.
In FIG. 1, reference character 1a designates the linear portion of an original to be read placed on an unshown transparent original supporting table lying at a regular position. The area from the end portion 1a' to the central portion 1a" of the linear portion 1a to be read which is long in S.sub.1 direction (major scanning direction) is projected and imaged upon the light-receiving area of CCD 5a by a lens 4a, and the area from the central portion 1a" to 1a"' is projected and imaged upon the light-receiving area of CCD 5b by a lens 4b. The light-receiving areas of CCDs 5a and 5b are linear and long in S.sub.1 direction, and CCDs 5a and 5b electrically scan the image in this direction and convert the optical image into electrical signals. That is, CCD 5a reads the area from 1a' to 1a" and CCD 5b reads the area from 1a " to 1a"'. As is well-known, an image pick-up element such as CCD has a light-receiving area comprising a number of picture elements (point-like photoelectric conversion sections) arranged in the major scanning direction, and electrical signals formed by a number of picture elements are successively taken out with respect to the major scanning direction. The original and CCDs 5a, 5b are moved relative to each other in S.sub.2 direction (minor scanning direction) perpendicular to the plane of the drawing sheet of FIG. 1 and the CCDs read the original from one end to the other with respect to the minor scanning direction. In FIG. 1, the optical axis Xa of the lens 4a enters the center of the area between points 1a' and 1a" and the optical axis Xb of the lens 4b enters the center of the area between points 1a"' and 1a". Reference characters 9a and 9b designate principal rays pointing to CCD 5a from points 1a' and 1a", and reference characters 9c and 9d denote principal rays pointing to CCD 5b from points 1a" and 1a"' . As is apparent from the foregoing, all of the angles formed by the principal rays 9a, 9b, 9c and 9d with the optical axes Xa and Xb, respectively, are .theta.. In other words, the angles of view of the lens 4a to the points 1a' and 1a" and the angles of view of the lens 4b to the points 1a" and 1a"' are all equal.
The above-described construction has the following inconveniences.
The imaging principal ray generally has an oblique angle with respect to the surface of the original and therefore, if the original supporting table flexes due to the weight of a book or the like thereon or the original floats up from the original supporting table, lack or overlap of information occurs in the boundary 1a" of the linear portion to be read. This will be described with reference to FIG. 1. In FIG. 1, reference character 1a indicates the regular position of the original, reference character 1b the position of the original when the original supporting table, not shown, has flexed, and the reference character 1c the condition of the original which has floated up from the original supporting table. When the original supporting table, not shown, flexes, the original assumes the position 1b. However, the positional relation between the imaging lenses 4a, 4b and the solid state image pick-up elements 5a, 5b does not change and the imaging principal rays 9a, 9b, 9c and 9d at the maximum angle of view form the same angle as that when the original is placed at the regular position 1a and thus, in the position 1b of the original, the portion A of the original is not read by the element 5a or 5b. Thus, the information which has existed in the portion A of the original is missing from the formed electrical signals.
On the other hand, when the original floats up from the original supporting table and assumes the position 1c, the information which has existed in the portion B of the original 1c is read by both of the elements 5a and 5b.
Also, there is a device in which, when the illumination of the original on the points 1a', 1a"' side is lower than the illumination at the central portion of the original, the lens 4a is disposed at a position more toward the point 1a' than the position of FIG. 1 and the lens 4b is also disposed at a position more toward the point 1a"' than the position of FIG. 1, whereby an attempt is made to make the illuminations at the opposite ends of the image pick-up elements 5a, 5b into the same degree, but in this case, the width of the areas A and B becomes greater than that in FIG. 1.